"And this wheel has fifteen weights on it. They're spaced equally around the rim, and the wheel's revolving at high speed."
Forrester kept his eyes closed. When he had the wheel nicely spinning, he said: "Okay. Now what?"
"Well," Diana said, "as long as the weights stay in place, the wheel spins evenly. But if you remove one of the weights, the wheel's out of balance. It starts to wobble."
Forrester took one of the weights (Dionysus, a rather large, jolly weight) off the wheel in his mind. It wobbled. "Right," he said.
"It can take the wobble for a little while. But unless the balance is restored in time, the wheel will eventually break."
Hurriedly, Forrester put Dionysus back on the wheel. The wobble stopped. "Oh," he said. "I see."
"Our power machine works in that sort of way. That is, it requires all fifteen occupants. Dionysus has been dead for three years now, and that's about the outside limit. Unless he's replaced soon, the machine will be ruined."
Forrester opened his eyes. The wheel spun away and disappeared. "So you found me to replace Dionysus. I had to look like him, so the mortals wouldn't see any difference. And the psychological similarity—"
"That's right," Diana said. "It's the same as the wheel again. If you remove a weight, you've got to put back a weight of the same magnitude. Otherwise, the wheel's still out of balance."
"And since the power machine works through the nervous system—"